Much work has been done over the last decade on self-assembled monolayers (SAMs) created on substrates like silicon, silicon dioxide, silver, copper or gold. Research on thiol monolayers on gold surfaces has resulted in technologies such as soft lithography. Applications of SAMs include sensor development, corrosion protection and heterogeneous catalysis. SAMs have been used as templates for organic synthesis and layer-by-layer adsorption. Their interaction with cells and proteins is well understood and micro-structured SAMs have been used to manipulate cells. All these techniques are based on a common approach: spontaneous monolayer formation of thiols on gold was used to achieve a densely packed two-dimensional crystal which offers reactive head groups for further modification. Chemisorption of thiols on gold occurs as a self-driven process and the packing of the thiols is mainly determined by geometrical aspects. Therefore, the monolayer is thought to be a closely packed layer with its alkyl chains being tilted relative to the surface at a specific tilt angle. An unmodified alkanethiol monolayer has a tilt angle between 20 and 30°. The distance between single alkyl chains is typically 5 Å (Ulman, Formation and Structure of Self-assembled Monolayers, Chem. Rev. 96, 1533–1554, 1996). Due to the spatial limitations and hydrophobic interactions between the alkyl chains, surrounding guest molecules are not expected to migrate into the monolayer. Rather, the head groups are thought to be the frontier line that determines interactions with the surrounding medium. For certain applications in electrode or sensor development, however, a monolayer with an adjustable degree of transparency for small chemical species might be desirable. It is thus desirable to control the density of the monolayer to produce membrane-like structure with a porosity of nanometer-scale is thereby created.
Self-assembled monolayers have been used to control and pattern the properties of a variety of surfaces. However, there is very little research concerning controlled switching between different surface properties. Okano's group reports that the wetability of a surface may be controlled by changing the temperature around the lower critical solution temperature (LCST) of poly(N-isopropylacryl)-grafts (Takei, et al., Dynamic Contact Angle Measurement Of Temperature-Responsive Surface Properties For Poly(N-Isopropylacrylamide) Grafted Surfaces. Macromolecules 27, 6163–6166, 1994). Lahann has also reported switching the surface properties of a stable fixed layer bound to a substrate (German Patent Publication 199 05 792, published Aug. 17, 2000). Accordingly, it is desirable to develop a method by which the surface properties of a self-assembled monolayer or similar structure may be reversibly switched upon application or removal of an external force field. The microscopic physico-chemical properties of the surface depend on the molecular structure of the interface of the surface with its environment (DeGennes, Wetting: Statistics And Dynamics. Rev. Mod. Phys. 57, 827–863, 1985). Applications such as microfluidic, bioseparation, optical displays, and sensors may benefit from techniques that manipulate the molecular composition of a surface.
Since the wetting behavior of a flat solid substrate is defined by the molecular-level structure of the interface, diverse approaches have been used to control wetting behavior (Mittal, Polymer Surface Modification: Relevance To Adhesion, VSP, Utrecht, 1996; De Crevoisier, et al., Switchable Tackiness And Wettability Of A Liquid Crystalline Polymer. Science 285, 1246–1249, 1999; Ichimura, et al., Light-Driven Motion Of Liquids On A Photoresponsive Surface. Science 288, 1624–1626, 2000; Abbott, et al., Reversible Wettability Of Photoresponsive Pyrimidine-Coated Surfaces. Langmuir 15, 8923–8928, 1999; Chaudhury, et al., How To Make Water Run Uphill. Science 256, 1539–1541, 1992). Temporal control of wetting properties has been demonstrated by elegant methods based on electrochemical surface modifications (Sondag-Huethorst, et al., Potential-Dependent Wetting Of Electroactive Ferrocene-Terminated Alkanethiolate Monolayers On Gold. Langmuir 10, 4380–4387, 1994; Byloos, et al., Phase Transitions Of Alkanethiol Self-Assembled Monolayers At An Electrified Gold Surface. J. Phys. Chem. B 105, 5900–5905, 2001; Iannelli, et al., Adsorption Of Pyrazine At The Au(111)/Aqueous Solution Interface. J. Electroanal. Chem. 376, 49–57, 1994) such as reversible oxidative desorption of surfactants (Gallardo, et al., Electrochemical Principles For Active Control Of Liquids On Submillimeter Scales. Science 283, 57–60, 1999). These systems require chemical reactions in order to control surface wettability. It is desirable to dynamically control surface wettability without relying on chemical reactions.